Radiation attenuation techniques have been used to measure or sense a parameter of a medium such as thickness, density, presence of a part, etc. Such techniques generally involve the passage or placement of the medium between a source of radiation and a radiation detector located in the shadow of the medium. The radiation emitted by the source thereof will then have to pass through the medium in order to be detected by the detector. The radiation is selected to be of a type that will be attenuated as a function of the parameter to be measured or sensed whereby the amount of radiation reaching the detector will vary as a function of such parameter.
One common type of radiation detector or probe used for sheet metal gauging applications includes a scintillation crystal which is optically coupled to a photomultiplier tube. The scintillation crystal converts impinging invisible radiation to bursts of visible light which are converted by the photomultiplier tube to electrical charge pulses. The charge pulses outputted by the photomultiplier tube are processed by electronic circuitry, such as a nuclear instrumentation module, which provides, for example, pulse count data to an associated display or system controller.
In sheet gauging systems wherein the radiation is attenuated as a function of sheet thickness, the rate at which scintillations are produced in the detector by such radiation will also be a function of sheet thickness. If only the scintillations caused by the attenuated radiation result in pulse count signals, the rate of such pulse count signals likewise would be a function of sheet thickness--the higher the rate, the thinner the sheet.
The sensing speed of the gauging system or other parameter measuring systems employing radiation attenuation techniques is related to the flux level or density of the radiation. That is, system sensing speed can be increased by increasing the flux level of the radiation which is to be attenuated as it passes through the medium for detection by the detector. Detectors or detector systems previously used in gauging applications, however, could not be used successfully with high radiation flux levels because of their limited dynamic range. For example, detector systems employing pulse counting circuitry are limited by the problem of pulse pileup, i.e., saturation of the electronic circuitry at high rates of detected scintillations. Consequently, this limited system sensing speed which in turn limited overall system speed such as the rate at which the sheet could be processed as in a sheet manufacturing and/or processing line.
Also known are detectors or detector systems which utilize current-to-frequency conversion circuitry. Such circuitry would operate to produce count signals at a frequency proportionally representative of the current output of a photomultiplier tube. Typically, this was accomplished by a current integrator which produced an electrical voltage output proportionally representative of the integral of the current output of the photomultiplier tube. When the integrator output reached a predetermined level indicating accumulation of a certain amount of charge in the feedback capacitor of the current integrator, a field effect transistor switch would be triggered to discharge the feedback capacitor, i.e., reset the capacitor to a ground or base line reference potential, and thereby generate a pulse count signal. One problem with such current-to-frequency conversion circuitry was that there was a certain amount of dead time when such circuitry was not responsive to the current output of the photomultiplier tube, such dead time being the time needed to discharge the capacitor to produce the pulse count signal. Consequently, there was a loss of stability at high current input rates as when the rate at which the feedback capacitor is being charged is on the order of the discharge rate of the feedback capacitor. Accordingly, such circuitry had limited dynamic range.